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Thursday, April 14, 2011

Fukushima Daiichi: Reactor cores

We've just seen a spokesman for a high-level group convened to consult TEPCO in plans of action at Fukushima Daiichi make a comment that might be misconstrued, or else is poorly translated: The quote essentially was that the damaged cores have dropped to the bottom of the pressure vessels in the Fukushima Daiichi plants.

This is probably both a gross oversimplification, and incorrect. At the moment, it is a practical certainty that the reactor cores are still entirely within the reactor pressure vessels. Upper portions of the fuel are surely demolished, along with the upper parts of the control rod blades (whose melt temperature is below that of the fuel rods.) Surely portions of this demolished core material, some of it melted and fused together into what we call "corium," have dropped to the bottom of the normal core area, onto the core support plate.

More than this is not certain, and would be hard to ascertain without better data than we have right now. It seems likely that cooling was maintained longer in No. 2 and No. 3, so that if any of the plants were likely to have experienced a core support plate failure -- meaning the core physically can drop to the bottom of the pressure vessel as the official is quoted as saying -- it would be No. 1 plant. However, there is no SOLID evidence of melt-related failure of any of the low penetrations or of the lower head.

Cooling water is getting out of the pressure vessels, and the location of this is not known exactly and frankly might be multiple locations. Salt buildup inside the reactor pressure vessels, and probably any and every pipe used to carry seawater during the time that seawater was all that was available (after having exhausted all feed water, condensate water, all fresh water stored in condensate surge tanks, and borated water SSD tanks) is now salted up to some degree. One estimate guessed that there might be as much as 25 tons of salt "cooked out" inside No. 1 reactor's pressure vessel and close-in piping. Surely some, or much of each core is encased in salt of some thickness.

The spike in radiation after the recent aftershock may have been the result of further sudden physical derangement of one of the reactor cores.

Beyond this any specific details of actual core conditions are a very risky thing to attempt to assess with certainty. Frankly, any further than this is really quite irrelevant at the moment since the real focus is on getting cooling water flow through the pressure vessels. The melted and deranged core elements will surely hinder water flow and cooling, leading to a random array of hot spots, and difficulty in eventual removal of the fuel. But for now, at least at far as the actual core conditions beyond what we've just described and the oft-repeated TEPCO estimates of percent fuel damage there isn't much more that could be said with any credence.